In the use of pressure wave superchargers as supercharge air compressors for internal combustion engines, the drive of the pressure wave supercharger rotor in the currently practiced state of technology takes place at a constant transmission ratio from the engine itself. The rotational speed of the rotor is therefore proportional to the engine rotational speed. The drive elements conventionally used for this purpose are V-belts, V-belt pulleys and belt tensioners. Although this type of drive has been proven in practice, there are still extra requirements, mainly because of the high transmission ratio between the engine and the pressure wave supercharger and the associated high belt and bearing peripheral speeds and also because of the severe transverse bearing loads due to the belt tension. The forces from the engine vibrations transferred via the belts to the pressure wave supercharger also contribute to the bearing loads. The pressure wave superchargers intended for the engines of passenger cars are also fairly small at relatively high engine powers. They do not therefore provide sufficient installation space for large rolling contact bearings capable of carrying heavy loads instead of the currently used rolling contact bearing sizes, whose dimensions have to be kept as small as possible for the reasons given below and whose rolling bodies therefore often run even without an inner race directly on a hardened section of the rotor shaft. These bearing designs make it possible to keep the rotor with its pressure exchange cells as small as is permitted by the exhaust gas and air throughput necessary for a specified power range of an engine. The smaller the shaft, bearing and thus also the rotor hub can be designed, therefore, the smaller the external dimensions of the rotor and hence of the complete pressure wave supercharger can be kept. In the case of direct mechanical drive of the rotor by the engine, however, lower limits are set to the bearing dimensions by the abovementioned factors of high transmission ratio, high bearing peripheral speed (and by the large amount of frictional heat caused by the high bearing peripheral speeds, which is difficult to remove because of the compact dimensions) and, especially because of the severe transverse loading due to the drive belts and the engine vibrations. The requirement for a particular minimum life of the bearings is thus a factor opposing further reduction of the bearing dimensions in this drive concept.
If the belt tension is avoided or if no use is made at all of any other driving element which causes transverse loading on the bearings, the mechanical and hence also the thermal bearing loads are reduced and, consequently, the bearing life is increased for the currently usual bearing dimensions. In a particular case, the bearings can be dimensioned so as to economize in space at a life which is otherwise the same. The transverse loading can, for example, be avoided by direct co-axial coupling of the rotor shaft to an electrical, hydraulic or similar drive source and, of course, also by coupling to an intermediate drive which accepts the transverse load due to the belt tension or the like and whose shaft has a coupling flange at the supercharger end and, at the other end, as usual preferably a belt pulley. Chain and gearwheel drives are practically out of the question as drive because of the large transmission ratios between the engine crankshaft and the rotor shaft of the pressure wave supercharger.
However, the abovementioned drives with intermediate drive, which are free from transverse forces, are also associated under certain circumstances with a decisive disadvantage for practical application. This is the limited freedom in the choice of the arrangement of the pressure wave supercharger in the engine compartment of a motor vehicle. In the case of belt drive, with or without intermediate drive, for example, the rotor shaft must be located parallel to the engine crankshaft--if unfavorable angle drives are not to be accepted. Electrical and hydraulic drives offer more freedom in this respect but are in turn more complicated, therefore more expensive and are also more difficult to deal with from a control point of view. Because the belt drive, together with the belt tensioning device, occupies one belt plane in the engine compartment, it makes the accommodation of the belt drive for the other auxiliaries more difficult. Full freedom in the arrangement of a pressure wave supercharger at economically justifiable cost is therefore provided, in practice, only by exhaust-gas drive, as in the case of exhaust-gas turbochargers. The supply of the exhaust gases to the exhaust gas casing of the pressure-wave supercharger, with its control edges and ports, can be carried out without any problems by an easily designed exhaust-gas duct and many possibilities are conceivable for converting the flow energy of the exhaust gases into the rotational motion of the rotor. Examples of such possibilities are tangential action on the rotor cell walls or on rows of blade rings specially provided for this purpose in association with elements fixed to the casing for the deflection and, if required, locally distributed concentration of the exhaust-gas flow into the rotor casing or into a ring of blading provided for that purpose in order to generate a tangential component of the inlet velocity of the exhaust-gas flow.
As a further disadvantage of the positively driven pressure wave supercharger, which has to be avoided, is the risk of a crack in the belt which demands expensive measures for emergency operation which must still ensure safe return home of the vehicle under its own power.
There is also difficulty in controlling the belt tension, which can be too large or too small and can therefore overload the bearing or lead to slip and, in consequence, premature belt wear. Because of the high rotational speed of the pressure wave supercharger, the problem of low pressure scavenging arises in the higher idling speed range of the engine and, similarly, incorrect flows to the rotor with corresponding inlet flow losses occur due to the fixed drive transmission ratio in certain operating ranges.
As already stated, an important advantage of the exhaust-gas-driven pressure wave supercharger with rolling contact bearings is that it can be arranged in any given position relative to the engine, including transversely or at any given angle or even at right angles to it.
Still further advantages should, however, be mentioned. The disappearance of the transverse load on the bearings permits the use of smaller bearings; in consequence, as also already mentioned, the peripheral velocity of the rolling bodies and hence, at the same rotational speed, the thermal loading becomes smaller relative to pressure wave superchargers with larger bearings. The smaller bearing diameters make it possible to increase the free cell cross-section for the same external dimensions of the supercharger, i.e. to increase the usable flow cross-section of the rotor. The supercharger can therefore be used for a larger engine. Advantages also appear with respect to the critical rotational speed, which appears at a greater distance from the rotational speed range at which the supercharger is mainly operated. Similarly, the response behavior becomes better during all acceleration procedures, provided the charging limit of the rotor is not exceeded. At low full-load rotational speeds of the engine, the exhaust-gas-driven rotor will also run more rapidly than one driven at a fixed transmission ratio from the engine. This reduces the pulsation of the charge air supply otherwise present at low rotational speeds and permits a smaller receiver volume, which in turn reduces the thermal inertia and makes the exhaust-gas receiver cheaper.
Pressure wave machines whose rotor is driven by a gas which has to be expanded independently of a prime mover are known from the patent literature. In contrast to the pressure converters mainly used as charging machines and which convert the energy contained in the exhaust-gas flow of an engine to increase the pressure of a supercharge airflow of approximately the same weight, the pressure of this supercharge air flow being above that of the exhaust-gas flow, the patents mentioned concern pressure exchangers, in which the air to be compressed is brought approximately to the pressure of the expanding medium, i.e. the exhaust gases of an engine, for example, or, in the case of the use of a pressure exchanger as the high pressure stage of a gas turbine, air heated in a combustion chamber or by a heat exchanger. In such a pressure exchanger, the energy of the exhaust gases or of the heated air serves to compress a quantity of cold air which is larger than the quantity of the exhaust gases or heated air to be expanded. Since in an engine, the supercharge airflow is approximately equal to the exhaust-gas flow, there is generally no requirement for the surplus air so that pressure exchangers are scarcely considered for supercharging but are considered, as stated, as the high pressure compressor or gas turbine in association with a conventional axial or centrifugal compressor as the low pressure compressor part and also for refrigerating machines, heat pumps, chemical processes, pressure fired steam boilers, etc.
Such a pressure exchanger is known from Swiss Patent No. 225,426. In the rotor of this pressure exchanger, the cell walls are inclined relative to an axial section plane, approximately in the form of a helical surface, or are curved in blade shape. The actual desired objective of cell walls formed in such a way consists in avoiding or reducing the shock losses of the gases taking place in the pressure exchange process at entry into or outlet from the rotor cells. The absolute outlet velocity relative to the rotor receivers a peripheral component so that the absolute outlet velocity is reduced; in contrast, the inlet of the gas can take place with shock, which drives the rotor.
A further pressure exchanger, which is located as the high pressure compressor between a low pressure axial compressor and a gas turbine and whose rotor can be either coupled to the turbine shaft or provided with its own drive independent of the gas turbine, is described in Swiss Patent No. 550,937. There is no mention of self-drive in this patent specification. It describes, rather, how the pressure difference between the expanding hot gas and the cold gas to be compressed in the low pressure zone can be increased by means of a special design of the rotor cells without simultaneously increasing the corresponding pressure difference on the high pressure side, in order to unload the compressor and, by this means, to increase the useful power and efficiency of the installation.
A pressure exchanger with self-drive by the pressure transmitting medium is, on the other hand, described in British Patent No. 921,686. In this, the cell walls on the inlet side are curved over one third of their length but are parallel to the axis in the other part and the associated inlet ports are inclined relative to the end surface of the rotor in such a way that they enter tangentially into the curved section of the rotor cells. The force driving the rotor arises due to the deflection of the inflowing medium on the curved part of the cell walls of the rotor.
The previously mentioned pressure wave machines are pressure exchangers which, as stated, can hardly be considered for the supercharging of internal combustion engines. This is reserved for pressure wave superchargers acting as pressure converters, in which achievement of the drive by the engine exhaust gases requires a series of measures which extend beyond the shaping of the cell walls of the rotor to deflect or change the direction of the exhaust gas flow and which may not previously have been proposed because a usable concept of a pressure converter which satisfies the operational requirements to be met by a supercharging unit is not known. There should, therefore, be hardly anything to discover in the relevant state of technology. A practically usable pressure wave supercharger with self-drive includes inter alia, a starting valve device, by means of which satisfactory engine starting and accelerating from rest under load in the cold condition, restarting the hot engine and driving away under load without delay is possible. It must ensure that the rotor supplies the supercharge airflow necessary for a sufficiently large acceleration torque and that for the particular part-load. Acceleration difficulties of the pressure wave supercharger in the case of a cold engine are caused by the fact that the grease in the rolling contact bearings is still stiff and/or by dirt in the rotor and hence increased friction between the casing and the end surfaces of the rotor. Such a starting valve, in association with other elements, also has to ensure satisfactory low idle running because otherwise, the rotor rotational speed would be so low that the particles contained in the exhaust-gas flow could pass over onto the air side. Devices such as throttle valves, wastegate and the like matched to the characteristic of the free-running pressure converter have to be provided for the control of the supercharge airflow over the whole of the load range.
In principle, the exhaust gas flow can be used without further measures to drive a rotor with cell walls parallel to the axis because of the swirl flow always present. This "natural" swirl flow is not, however, capable of accelerating the rotor sufficiently rapidly and to sufficiently high rotational speeds corresponding to the particular load conditions.
As stated and as appears from the relevant publications discussed, the known relevant means for driving a free-running rotor consist of cell walls curved or oblique to the rotor axis or cell wall parts in association with exhaust-gas inlet flow ducts correspondingly inclined to the rotor axis for increasing the velocity component of the exhaust-gas flow acting in the peripheral direction of the rotor. These means alone, however, do not suffice in their known form to meet the requirements placed on an actual pressure wave supercharger. They therefore have to be adapted to these requirements, more effectively designed and supported by elements to concentrate the exhaust-gas flow. Since the exhaust-gas flow energy necessary to drive the rotor is not, of course, available for the compression work to be transferred to the air, the increase in the leakage gaps (caused by the different amounts of heating and the different material expansion coefficients of the rotor and casing) between the end surfaces of the rotor and the gas and air casings during unsteady state operating conditions, such as starting and acceleration, must be kept as evenly small as possible in the interest of good compression efficiency.
The present invention achieves the objective of producing an exhaust-gas-driven pressure wave supercharger acting as a pressure converter, which pressure wave supercharger satisfies the requirements sketched above and avoids the disadvantages described of the pressure wave supercharger driven at a constant transmission ratio by the engine.